Writing a SSG for oneself is just like writing a single Makefile, for one small project, with one contributor, without cross platform concerns.

Using a generic SSG is like learning CMake.

This is quite well put.

Althought most of the times using a generic SSG feels more like learning the autotools than CMake.

This.

The complexity in most SSGs comes from them being generic solutions.

If you have a fairly simple site and know what you need, you probably get it build with a few lines of JS and a GH action.

Any example?